feat: support for binary key values

[akafredperry]

closes #369

- What I did

• Implemented the AtClient methods for getting and putting binary values (byte[]).
• Fixed a bug in EncryptionUtils (no longer sensitive to JVM default charset)
• Added Netty to list of dependencies in README (missing from previous commit)
• Added additonal BDD tests for binary values API

- How I did it

• Implemented Base2e15 codec
• Integrated this utility code into AtClientImpl
  NOTE: put methods will always set isBinary=true in the update, and get methods will always check isBinary=true in the metadata. It is legitimate to invoke get() for a binary key value but NOT to invoke getBinary() for a string key value.

- How to verify it
PR contains

• Unit tests which verify the Base2e15 implementation. Those unit tests replicate the single test in the dart implementation, specifically testing with the same input and expected output. They also include a test that does 1000
  iterations of increasing byte array length with random bytes, testing that decoded encoded result matches the original input. And then there are tests that explicitly test repeating sequences of 14, 15, and 16 byte arrays (either side of the 15 bit boundary).
• Unit tests for AtClientImpl (previously this was only ever testing via the BDD integration tests).
• New BDD tests that demonstrate AtClient put() with binary values and getBinary()

All of these tests will be run as part of this PR pipeline.

- Description for the changelog
support for binary key values

[gkc]

LGTM but have you done cross-platform checks? (Java putter -> Dart getter, and vice versa)

[akafredperry]

LGTM but have you done cross-platform checks? (Java putter -> Dart getter, and vice versa)

good point, I will do this, thanks